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114SYNTHESIS OF THE T^AlC AND Ti3AlC2 MAX PHASE WITH A REDUCTION STEP VIA COMBUSTION OF A TiO2 + Mg + Al + C MIXTURE
V. I. Vershinnikov*" and D. Yu. Kovalev"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, 142432 Russia *e-mail: vervi@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10185
The last decade has seen increasing interest in the synthesis and characterization of MAX phases—ternary compounds with a hexagonal close-packed structure and the general formula Mn + 1AXn, where M is a transition metal, A is a group IIIA or IVA ^-block element (for example, Si, Ge, Al, S, Sn, and others), X is carbon or nitrogen, and n = 1-4. The MAX phases have a layered crystal structure in which [Mn + 1Xn] carbide or nitride slabs are sandwiched between monolayers of group IIIA or IVA elements. Their layered crystal structure leads to a laminate structure of grains, with layer thicknesses of up to several tens of nanometers. Materials based on the MAX phases combine properties of ceramics and metals. Like ceramics, they have low density and high elastic moduli, heat resistance, and high-temperature strength. Like metals, they have appreciable electrical and thermal conductivity and offer high fracture toughness and thermal stability. To date, about 70 compounds of the MAX family have been reported. The best studied MAX phases, which are of practical interest, are the titanium-based MAX phases: Ti2AlC and Ti3AlC2. Such compounds are typically prepared from elemental mixtures by hot isostatic pressing (HIP) [1], spark plasma sintering (SPS) [2], and self-propagating high-temperature synthesis (SHS) [3, 4]. Despite the diversity of synthesis processes, almost all of them use oxygen-free compounds as starting materials. Li et al. [5] prepared the Ti2AlC and Ti3AlC2 phases using a titanium oxide, aluminum, and graphite. The MAX phases Ti2AlC and Ti3AlC2 were synthesized by heating a mixture of the starting materials to a temperature of 1500°C, followed by isothermal holding for 2 h. EXPERIMENTAL
In our experiments, we used the following powders: magnesium (98.5-99.5% purity, particle size under 250 p,m), commercial TiO2 (grade 1, Russian Federation State Purity Standard TU 1715-347-00545484-94), aluminum (99.5% purity, particle size under 5 p,m), carbon black (P804-T, specific surface area S = 12 m2/g), and graphite (S = 3.6 m2/g). MAX phases were isolated from intermediate products (Ti2AlC MgO and Ti3AlC2 MgO) using dilute hydrochloric acid (1:3). Mixing was performed in tumbling mills. We used samples with a loose bulk density. A starting mixture weighing 250 g was poured into a graphite boat. Combustion was initiated on the top surface of the samples using a tungsten coil. The combustion process was run in an SVS-8 reactor under an argon atmosphere at a pressure P = 4 MPa. RESULTS AND DISCUSSION
Ti2AlC MAX phases were synthesized in the combustion regime in an argon atmosphere under a pressure of 3 MPa according to the reaction scheme:
2TiO2 + 4Mg + Al + C = Ti2AlC + 4MgO (1)
After chemical leaching in hydrochloric acid, the powder consisted of Ti2AlC, MgAl2O4, and TiC (Table 1, run 1). The formation of MgAl2O4 (spinel) points to deficiency of the reducing agent (magnesium) kin the starting mixture. As a result, some of the aluminum reduces
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the titanium dioxide to give titanium metal and AhO3. This leads to the formation of MgO-AhO3.
Table 1 presents the ratio of the starting materials and the phase composition of the powder after acid enrichment of the Ti2AlC-MgO and Ti3AlC2^MgO intermediate product in hydrochloric acid.
Table 1. Ratio of the starting materials and phase composition of the powder after acid enrichment of the Ti2AlC-MgO and Ti^AlC^ intermediate product in hydrochloric acid._
Composition, wt % Phase composition of the powder, wt % Run no.
TiO2 Mg Al C Ti2AlC TisAlC TiC MgAl2O4
54.2 32.6 9.2 4 28 0 55 17 1
50.9 36.7 5.6 3.8 90 0 10 0 2
50.5 36.4 9.3 3.8 80 0 13 7 3
49.4 38.6 8.4 3.6 93 0 7 0 4
48.6 39.5 8.3 3.6 87 0 13 0 5
49.6 38.8 8.4 3.2 96 0 4 0 6
49.1 38.3 8.3 4.3 11 86 3 0 7
48.9 38.2 8.3 4.6 4.6 89.4 6 0 8
Increasing the magnesium content of the starting mixture to Mgex = 20% leads to complete magnesium reduction of the titanium dioxide and the formation of the MAX phase Ti2AlC and titanium carbide (Table 1, run 2). The addition of 10% excess of aluminum to this starting mixture led to an increase in the percentages of MgAhO4 (spinel) and titanium carbide in the Ti2AlC powder (Table 1, run 3). Increasing the excess of magnesium in the starting mixture to 30% led to a decrease in the percentage of titanium carbide in the final product. After leaching, the powder consisted of the MAX phase Ti2AlC and a small amount (7%) of titanium carbide (Table 1, run 4). Increasing the excess of magnesium in the starting mixture to 35% leads to an increase in the percentage of titanium carbide in the powder of the MAX phase Ti2AlC. In subsequent investigation, we used 30% excess of magnesium (Table 1, run 4) in the starting mixture. We studied the effect of carbon black deficiency on the formation of the MAX phase Ti2AlC and the titanium carbide content of the powder. Reducing the carbon black content of the starting mixture by 10 wt % leads to a decrease in the titanium carbide content of the powder by 4% (Table 1, run 6). A 20% carbon black deficiency leads to a sharp increase in titanium carbide content and a decrease in the content of the MAX phase Ti2AlC in the powder. Figure 1a shows the X-ray diffraction pattern of the Ti2AlC powder (Table 1, run 6) and Figure 1b shows its micrograph. It is seen from the X-ray diffraction pattern that the powder consists of two phases: Ti2AlC and TiC.
(a) (b)
Fig. 1. (a) X-ray diffraction pattern and (b) micrograph of the Ti2AlC powder (Table 1, run 6: Mgex = 30%, Cdef = 10%, Pa = 3 MPa, loose bulk density of the starting mixture).
Increasing the excess of carbon black in the starting mixture to 20% created conditions for the formation of a mixture of the MAX phases Ti2AlC and Ti3AlC2 and titanium carbide (Table 1, run 7). The X-ray diffraction pattern of this powder is presented in Fig. 2.
2©, degree
Fig. 2. X-ray diffraction pattern of the Ti2AlC + Ti3AlC2 + TiC powder (Table 1, run 7: Mgex = 30%, Cex = 20%, Pa = 3 MPa, loose bulk density of the starting mixture).
A further increase in the excess black carbon to 35% led to the formation of the powder consisting of the Ti3AlC2 phase and some amount of Ti2AlC and titanium carbide (ex. 8, Table 1).
Figure 3 demonstrates X-ray diffraction pattern and SEM image of the powder at 30% excess of carbon black in the mixture. MAX phase particles are thin plates of 70 nm or more in thickness.
(a) (b)
Fig. 3. (a) X-ray diffraction pattern and (b) micrograph of the powder (Table 1, run 8): Cex = 30%, Pa = 3 MPa, loose bulk density of the starting mixture.
CONCLUSIONS
It follows from the present results that the MAX phase Ti2AlC can be prepared by SHS with a reduction step. In this process, an excess of magnesium helps to minimize the percentage of MgAl2O4 (spinel), and carbon deficiency in the starting mixture leads to a decrease in the percentage of titanium carbide in the final product. An excess of carbon black in the starting mixture leads to the formation of a mixture of the MAX phases Ti2AlC and Ti3AlC2.
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